Rising levels of atmospheric carbon dioxide that are associated with global warming can interfere with plants' ability to incorporate certain forms of nitrogen, dramatically altering the plant life worldwide and forcing significant changes in agricultural fertilizer use, according to a plant physiologist at the University of California, Davis.

These findings will be reported by lead author Arnold Bloom and colleagues in the Feb. 5 issue of the Proceedings of the National Academy of Sciences.

"It's been known for some time that increased concentrations of carbon dioxide in the atmosphere initially boost carbon intake and growth in plants but eventually the accelerated carbon assimilation declines," said Bloom, a professor in the UC Davis vegetable crops department. "The results from our study indicate that carbon dioxide inhibition of nitrate assimilation contributes to this phenomenon and suggest two physiological mechanisms that may be responsible."

Atmospheric monitoring since 1800 indicates that carbon dioxide concentrations have risen by more than 30 percent during the past two centuries. For many years, scientists believed these rising levels of carbon dioxide would actually benefit plants because carbon dioxide is one of the essential ingredients in photosynthesis, the process by which green plants use sunlight to manufacture the chemical energy they need.

Further study, however, revealed that the accelerated rate of carbon dioxide assimilation wasn't sustained. In laboratory experiments, plants initially responded to a doubling of atmospheric carbon dioxide levels by assimilating 30 percent more carbon. But within a few days or weeks, this accelerated rate of carbon processing dropped back to just 12 percent greater than normal.

Against that backdrop, Bloom and colleagues have been studying how crop plants respond to being fertilized with two different forms of nitrogen: nitrate and ammonium. Nitrogen is an element that is key to producing proteins and nucleic acids such as DNA in plants. Because it is so important to plant growth, farmers and gardeners commonly apply nitrogen-rich fertilizers to their crops.

In this newly published study, the UC Davis researchers discovered that nitrate fertilizer is not nearly as efficient as ammonium fertilizer when atmospheric carbon dioxide levels are unusually high. In laboratory experiments they grew wheat seedlings with either nitrate or ammonium under varying concentrations of atmospheric carbon dioxide. They discovered that elevated levels of carbon dioxide inhibited the processing of nitrate in the wheat leaves in two ways.

First, plants place a higher priority on storing and processing carbon dioxide than they do nitrogen, so when carbon dioxide levels rose, some of the chemicals needed to assimilate the nitrate were already tied up in assimilating carbon dioxide.

Second, to make use of nitrate, the plants have to convert nitrate into nitrite and then move the nitrite into structures within their cells called chloroplasts, which are the center for photosynthesis. Bloom's research indicated that elevated levels of carbon dioxide interfered with the overall process of photosynthesis by blocking this vital transfer of nitrite into the chloroplasts.

Furthermore, the researchers found that wheat growth wasn't influenced by the type of nitrogen available as long as atmospheric carbon dioxide was at a normal level. However, when atmospheric carbon dioxide rose to nearly twice the normal level -- a level that is likely to be reached within the next century -- the leaves of plants receiving ammonium increased in size by nearly 49 percent, while plants receiving nitrate increased by only 24 percent. In short, the plants receiving ammonium responded much more to the increased carbon dioxide than did the plants receiving nitrate.

Additionally, the protein content of the wheat plants receiving ammonium increased 73 percent under elevated carbon dioxide compared to only 32 percent for the wheat plants receiving nitrate. These data suggest that rising atmospheric carbon dioxide levels might diminish the nutritional quality of grain receiving nitrate fertilizer.

"We expect that the data from this study will have real-world implications for crop production," Bloom said. "In well drained soils generally devoted to wheat production, nitrate is the common form of nitrogen available in the soil. This study suggests that a shift to increase ammonium availability might be needed."

This published study focused only on wheat, but the UC Davis researchers have since repeated the work using tomatoes and have documented similar results.

Bloom added that the study also suggests that plant and tree species in natural ecosystems that depend on nitrate conversion into amino acids in their leaves are likely to be at a competitive disadvantage to those species that are either able to convert nitrate into amino acids in their roots or use ammonium as their predominant nitrogen source. This may result in significant changes in the distribution of plants in the wild as atmospheric carbon dioxide levels continue to rise, he suggested.

This study was funded by the Department of Energy and the National Science Foundation.